Adaptive signal equalizer with segmented coarse and fine controls

ABSTRACT

Circuitry for adaptive signal equalizing with coarse and fine boost controls by providing multiple serially coupled stages of parallel controllable DC and AC signal gains with coarse and fine gain controls provided across all stages.

RELATED APPLICATION DATA

This application is a non-provisional based on and claiming priorityfrom U.S. Provisional Application No. 61/365,531, filed Jul. 19, 2010.

BACKGROUND

1. Field of the Invention

The present invention relates to interface circuits for receiving highdata rate signals from long lengths of cable, and in particular,interface circuits for receiving high data rate, baseband, binaryencoded data signals from long lengths of cable.

2. Description of the Related Art

In a typical high speed digital wire-line communication system, thechannel introduces frequency dependent loss. These losses causeinter-symbol interference (ISI) when the channel is conveying a randomdata pattern. An equalizer removes the ISI by implementing the inversechannel response that compensates for the signal distortion caused bythe channel. An adaptive equalizer automatically compensates for theloss of the channel.

Recovering data which has been transmitted over a long length of cableat high rates requires that such data be equalized in order tocompensate for the loss and phase dispersion of the cable. Further, inthose applications where the cable length may vary, such equalizationmust be based upon a complementary transfer function which is capable ofadapting accordingly since the transfer function of the cable varieswith the length of the cable. This equalizing is generally done usingthree functions: a filter function; a dc restoration and slicingfunction; and an adaptation control, or servo, function.

The filter function is performed using a complementary (with respect tothe complex cable loss characteristic) filter which synthesizes theinverse of the transfer function of the cable. Since the bit error rate(BER) is directly related to jitter, an important performance metric foran equalizer is jitter within the output waveform. The extent to whichthe equalizer is able to match the inverse of the complex cable losscharacteristic determines the extent to which inter-symbol interferenceinduced jitter is eliminated.

Conventional equalizers use gm/C types of continuous time filters orfinite impulse response (FIR) filters. However, these types of filterstructures tend to be complex and have difficulty maintaining therequired balance among the desired operating characteristics, such asoutput jitter, compensation for process and temperature variations, andoptimization of the signal-to-noise ratio (SNR).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an adaptive signal equalizer inaccordance with a preferred embodiment.

FIG. 2 is a functional block diagram of an exemplary embodiment of thehigh rate filter of FIG. 1.

FIG. 3 is a functional block diagram of an exemplary embodiment of thelow rate filter of FIG. 1.

FIG. 4 is a functional block diagram of an exemplary embodiment of theDC restoration and slicer stages of FIG. 1.

FIG. 5 is a functional block diagram of an exemplary embodiment of theadaptation stages of FIG. 1.

FIG. 6 is a functional block diagram of an alternative embodiment of theintegration and summing stages of FIG. 5.

FIG. 7 is a functional block diagram of an exemplary embodiment of asignal conversion stage for use as part of the adaptation stages of FIG.1.

FIG. 8 is a functional block diagram of an exemplary embodiment of thecontrol stage of FIG. 1.

FIG. 9 is a state diagram of an exemplary embodiment of an algorithmused by the finite state machine of FIG. 8.

FIG. 10 is a schematic diagram of an exemplary embodiment of theequalization circuits of FIGS. 2 and 3.

FIG. 11 is a partial schematic diagram of an exemplary embodiment of anAC portion of the equalization circuit stages of FIG. 10.

FIG. 12 is a diagram depicting an exemplary embodiment of a step-wiselinear control for the fine tuning of equalization.

FIG. 13 is a timing diagram of an exemplary embodiment of timing forcoarse and fine boost up equalization adjustments.

FIG. 14 is a timing diagram of an exemplary embodiment of timing forcoarse and fine boost down equalization adjustments.

FIG. 15 is a functional block diagram of an exemplary embodiment of therate detection stage of FIG. 1.

DETAILED DESCRIPTION

The following detailed description is of example embodiments withreferences to the accompanying drawings. Such description is intended tobe illustrative and not limiting with respect to the scope of allpossible embodiments. Such embodiments are described in sufficientdetail to enable one of ordinary skill in the art to practice them, andit will be understood that other embodiments may be practiced with somevariations without departing from the spirit or scope of the subjectinvention.

Throughout the present disclosure, absent a clear indication to thecontrary from the context, it will be understood that individual circuitelements as described may be singular or plural in number. For example,the terms “circuit” and “circuitry” may include either a singlecomponent or a plurality of components, which are either active and/orpassive and are connected or otherwise coupled together (e.g., as one ormore integrated circuit chips) to provide the described function.Additionally, the term “signal” may refer to one or more currents, oneor more voltages, or a data signal. Within the drawings, like or relatedelements will have like or related alpha, numeric or alphanumericdesignators. Further, while the present invention has been discussed inthe context of implementations using discrete electronic circuitry(preferably in the form of one or more integrated circuit chips), thefunctions of any part of such circuitry may alternatively be implementedusing one or more appropriately programmed processors, depending uponthe signal frequencies or data rates to be processed. Moreover, to theextent that the figures illustrate diagrams of the functional blocks ofvarious embodiments, the functional blocks are not necessarilyindicative of the division between hardware circuitry. Thus, forexample, one or more of the functional blocks (e.g., processors,memories, etc.) may be implemented in a single piece of hardware (e.g.,a general purpose signal processor, random access memory, hard diskdrive, etc.). Similarly, any programs described may be standaloneprograms, may be incorporated as subroutines in an operating system, maybe functions in an installed software package, etc.

An adaptive signal equalizer in accordance with one or more preferredembodiments includes one or more of a number of features. Adaptiveequalization can be provided with separate equalization boost andamplitude control loops. Adaptive equalization can also be provided withdifferent equalization characteristics depending upon whether a higheror lower data rate is received. Adaptive equalization can be furtherprovided using an initial binary search to reduce the number ofnecessary data points to be analyzed before reaching the desiredequalization, and may include an initial equalization setting (e.g.,based on control data stored in a lookup table). The equalizationcircuit architecture includes coarse control, and may also include finecontrol, along with means for controlling the transition between coarseand fine adjustments in the equalization.

Adaptation of the equalization is based on interlaced successiveapproximation of digital boost and amplitude codes. Energy detectionpoints are separated for high data rate and low data rate equalizationpaths. Different filter bandwidths are used for adaptation based on highand low data rates. Boost-dependant amplitude calibration provides ahigher calibration range. Power consumption and thermal noise arereduced in the equalization data paths compared to conventional analogadaptation techniques. Further power consumption and thermal noisereductions are achieved by avoiding the use of an automatic gain control(AGC) stage for DC amplitude calibration. Interactions between theamplitude and equalization boost control loops and deadlock are reduced.Linear equalization is segmented to allow for optimal equalization formultiple channels. Both coarse and fine equalization boosts areprovided, with appropriate timing when transitioning between coarse andfine adjustments and when increasing or decreasing the digital boostcodes. Data rate detection is provided to differentiate between high(e.g., 1.485 Gbps) and low (e.g., 270 Mbps) data rates, with such ratedetection used to control the adaptation algorithm. Separate filterbandwidths for high and low data rate paths minimize crosstalk andimprove noise performance independently.

Referring to FIG. 1, an adaptive signal equalizer 100 in accordance withone embodiment includes multiple stages interconnecting and interactingsubstantially as shown: a high (data) rate filter stage 102, a high rateDC restoration and slicing stage 104, a high rate adaptation stage 106,a low (data) rate filter stage, 112, a low rate DC restoration andslicing stage 114, a low rate adaptation stage 116, a signal multiplexor118, a rate detection stage 130 and a control stage 120. As discussed inmore detail below, high data rate signals are processed by the high ratefilter 102, high rate DC restoration and slicer 104 and high rateadaptation 107 stages, while low data rate signals are processed by thelow rate filter 112, low rate DC restoration and slicer 114 and low rateadaptation 116 stages (e.g., with the high rate filter stage 102 set forless equalization or unity signal gain with no equalization). Inaccordance with a rate detection signal 131 (which is indicative ofwhether the incoming signal 101 has a high or low data rate), themultiplexor 118 provides the equalized high 105 or low 115 data signalas the equalized output signal 119.

The high rate filter stage 102 provides controllable amounts ofequalization in accordance with high rate coarse 125 and fine 127control signals. The resulting equalized signal 103 is DC-restored andsliced by the DC restoration and slicer stage 104 in accordance with anamplitude control signal 121 (discussed in more detail below).

This equalized signal 103 is further equalized by the low rate filter112 in accordance with low rate coarse 129 and fine 127 equalizationcontrol signals (discussed in more detail below). The resultingequalized signal 113 is DC-restored by the DC restoration and slicerstage 114 in accordance with the amplitude control signal 121.

The first equalized signal 103 is also used by the rate detection stage130 to determine whether the incoming signal 101, as represented by thefirst equalized signal 103, has a high data rate or a low data rate. Itsoutput signal 131 is indicative of the data rate (e.g., high or low).

One of the DC-restored and sliced signals 105, 115 is selected by themultiplexor 118, in accordance with the rate detection signal 131, asthe equalized output signal 119. For example, if the rate detectionsignal 131 is indicative of an input signal 101 having a high data rate,the high rate equalized signal 105 is selected. Conversely, if the ratedetection signal 131 is indicative of the incoming signal 101 having alow data rate, the low rate equalized signal 115 is selected.

The high rate adaptation stage 106 processes the equalized input signal103 and DC-restored and sliced signal 105 of the first DC-restorationand slicer stage 104 to provide a feedback signal 107 to the controlstage 120 (discussed in more detail below). Similarly, the low rateadaptation stage 116 processes the low rate equalized signal 113 andDC-restored and sliced signal 115 of the second DC restoration andslicer stage 114 to provide another feedback signal 117 to the controlstage 120 (discussed in more detail below).

As discussed in more detail below, the control stage 120 receives andprocesses the adaptation feedback signals 107, 117 and rate detectionsignal 131 to provide the amplitude control signal 121, a reset signal123 and equalizer boost control signals 125, 127, 129.

Referring to FIG. 2, an exemplary embodiment of the high rate filterstage 102 includes four equalizer circuits 202 a, 202 b, 202 c, 202 dand a digital-analog converter (DAC) 202 e, interconnected substantiallyas shown. The incoming signal 101 is successively equalized by eachequalizer circuit 202 a, 202 b, 202 c, 202 d to produce the firstequalized signal 103. Each equalizer circuit 202 a, 202 b, 202 c, 202 dis controlled in accordance with a respective subset 125 a, 125 b, 125c, 125 d of the high rate coarse equalization control signal 125. Inthis exemplary embodiment, the 24-bit control signal 125 is split intofour respective 6-bit control signals. The fine equalization controlsignal 127 is converted by the DAC 202 e to an analog control voltageVfine 203 e for fine tuning the equalization performed by eachequalization circuit 202 a, 202 b, 202 c, 202 d (discussed in moredetail below).

In accordance with a preferred embodiment, these four equalizer circuits202 a, 202 b, 202 c, 202 d provide a total of 60 dB of maximum boost(e.g., 15 dB per circuit), using six coarse steps corresponding to 2.5dB boost per step, and 32 fine steps, thereby providing a resolution of0.08 dB. The coarse boost control signal 125 use a thermometer code, sothe fine boost signal 127 can be shared across all equalizer circuits202 a, 202 b, 202 c, 202 d, i.e., as the converted analog controlvoltage 203 e.

Referring to FIG. 3, an exemplary embodiment of the low rate filterstage 112 includes an equalization circuit 212 a and a DAC 212 b,interconnected substantially as shown. The first equalized signal 103 isfurther equalized by the equalization circuit 212 a to produce thesecond equalized signal 113. Coarse adjustment of the equalization is inaccordance with the low rate coarse control signal 129, while fineadjustment of the equalization is done in accordance with an analogcontrol voltage 213 b provided by the DAC 212 b based on the finecontrol signal 127.

This equalizer circuit 212 a includes seven internal stages (discussedin more detail below), resulting in seven coarse steps, each of which isfurther divided into 32 fine steps. As with the high rate filter stage102, the coarse boost follows a thermometer code, so the fine boostlines can all be driven by the same analog control signal 213 b.

Accordingly, in accordance with a preferred embodiment, the four stagesof equalization within the high rate filter 102 provides 768 fine steps(6*32*4=768), and the low rate filter stage 112 provides 224 fine steps(7*32=234), resulting in a total of 992 fine steps.

Referring to FIG. 4, an exemplary embodiment of circuitry to implementthe DC restoration and slicer stages 104, 114 includes respective onesof a slicer circuit 204 a/214 a, a bias current source 204 b/214 b forcoarse control, a bias current source 204 c/214 c (e.g., implemented ascurrent DACs) for fine current control, and a lookup table (LUT) 204d/214 d, all interconnected substantially as shown. As discussed above,the input signal 103/113 is DC-restored and sliced by the slicer circuit204 a/214 a to provide the DC-restored and sliced signal 105/115.Amplitude control of the output signal 105/115 is achieved bycontrolling the coarse Ic and fine If bias currents in accordance withthe coarse boost control signal 125 that addresses LUT current controldata 205 d/215 d, and fine amplitude control signal 121. respectively.During low data rate equalization, the fine amplitude control signal 121is held constant.

Referring to FIG. 5, an exemplary embodiment of the adaptation stages106, 116 includes band pass filters 206 a/216 a, 206 b/216 b, full waverectification circuits 206 c/216 c, 206 c/216 d, signal summingcircuitry 206 e/216 e, and integration circuitry 206 f/216 f,interconnected substantially as shown. The input 103/113 and output105/115 signals of the DC restoration and slicer stage 104/114 arefiltered by respective band pass filters 206 a/216 a, 206 b/216 b. Asdiscussed in more detail below, each filter 206 a/216 a, 206 b/216 b hasmultiple available bandwidths (e.g., two), one of which is selected inaccordance with a bandwidth control signal 133/135. The filtered signals207 a/217 a, 207 b/217 b are full-wave rectified by the rectificationcircuits 206 c/216 c, 206 d/216 d. The summing circuitry 206 e/216 e isused to find the difference between these rectified signals 207 c/217 c,207 d/217 d, with the resulting difference signal 207 e/217 e beingintegrated by the integration circuitry 206 f/216 f to provide theadaptation feedback signal 107/117.

Referring to FIG. 6, in accordance with an alternative embodiment, theordering of the subtraction and integration of the rectified signals 207c/217 c, 207 d/217 d can be reversed, as shown, with the rectifiedsignals 207 c/217 c, 207 d/217 d first being integrated and thensubtracted to provide the adaptation feedback signals 107/117.

Referring to FIG. 7, in accordance with a preferred embodiment, thecircuitry of FIG. 1 is implemented as differential circuitry withdifferential signals. Accordingly, the adaptation feedback signals 107,117 include respective positive 107 p, 117 p and negative 107 n, 117 nsignal phases which are converted by a differential-to-single-endedconversion circuit 302 when applied across an automatic equalizationcontrol (AEC) capacitance 304 to produce a single-ended adaptationfeedback signal 107/117. The reset signal 123 controls resetting of theaccumulated charge across the AEC capacitance 304 (discussed in moredetail below).

Referring to FIG. 8, an exemplary embodiment of the control stage 120includes a multiplexor 220 a and a finite state machine (FSM) 220 b,interconnected substantially as shown. Depending upon whether the inputsignal 101 is identified by the rate detection signal 131 as having ahigh or low data rate, the multiplexor 220 a selects either the high 107or low 117 rate adaptation feedback signal as the signal 221 a to beprovided to the FSM 220 b. In accordance with the selected adaptationfeedback signal 221 a, the a FSM 220 b provides the amplitude controlsignal 121, reset signal 123 and equalizer boost control signals 125,127, 129, and adaptation filter control signals 133, 135 (discussed inmore detail below).

Referring to FIG. 9, the finite state machine 220 b operates inaccordance with an algorithm 400 as follows. Following initialization402, an optimal equalizer boost is digitally selected using a binarysearch 404. As is well known in the art, for N programmable equalizerboost settings, it will take log₂ (N) search steps to find the optimalequalization boost. The state machine 202 b controls the sequentialresetting and integration of charge on the AEC capacitance 304 for eachstep in the binary search process and then updates the equalizationboost, i.e., to be higher or lower. Following completion of the binarysearch 404, the algorithm transitions 405 a to amplitude adjustment 406with the lower bandwidths of the filters 206 a, 216 a, 206 b, 216 b inthe adaptation stages 106, 116 selected. This lower bandwidth carriesthe amplitude information and a linear search is used for amplitude loopconvergence while performing amplitude adjustment 406. The amplitudes ofthe output signals 105, 115 of the DC restoration and slicer stages 104,114 are tuned to match the amplitudes of their respective equalizedinput signals 103, 113. This advantageously avoids the need of an AGCamplifier in the equalizer paths. The convergence of the amplitude loopis detected by a change in direction of the amplitude code 407,following which the state machine 220 b transitions 407 to boostadjustment 408 and the higher bandwidths of the band pass filters 206a/216 a, 206 b/216 b in the adaptation stages 106/116 are selected. Thefinite state machine 220 b then begins linear equalization boostadjustment 408. A change in direction or timeout in the equalizationboost loop causes the state machine 220 b to transition back 409 toamplitude adjustment 406.

In accordance with an alternative embodiment, following completion ofthe binary search 404, the algorithm can instead first transition 405 bto boost adjustment 408, with the higher bandwidths of the filters 206a, 216 a, 206 b, 216 b in the adaptation stages 106, 116 selected.

Upon convergence, the average value of the voltage across the AECcapacitance 304 (FIG. 7) will be zero for both amplitude (low bandwidth)and equalization boost (high bandwidth) frequency bands of the band passfilters 206 a/216 a, 206 b/216 b. The state machine 220 b will toggleback and forth 407, 409 between adjacent amplitude and equalizationboost settings that are finely spaced.

Referring to FIG. 10, an exemplary embodiment of the equalizer circuits202 a, 202 b, 202 c, 202 d, 212 a of the high rate filter 102 and lowrate filter 112 (FIGS. 2 and 3) include multiple stages ofparallel-connected DC amplifiers 502 and AC amplifiers 504 for receivingthe positive 101 p/103 p and negative 101 n/103 n phases of thedifferential input signal 101, and providing the positive 103 p/113 pand negative 103 n/113 n signal phases of the output signals 103/113which have been equalized as discussed above. The amplifiers 502, 504are biased from a power supply VCC through resistors having a value R.Both the DC 502 and AC 504 amplifiers use differentially coupled NPNbipolar junction transistors with emitter degeneration resistanceshaving a value 14R and bias current sources as shown. (It should benoted that this circuitry of FIG. 10 includes seven differentialamplifiers stages, reflecting the seven stages used by the low rateequalizer circuit 212. For each of the high rate equalizer circuits, 202a, 202 b, 202 c, 202 d, six amplifiers stages are used and the emitterdegeneration resistances have a common value of 12R.) The AC amplifiers504 also include tunable impedances Z1, Z2, Z3, Z4, Z5, Z6, Z7 (Z1-Z6for the high rate equalizers), which are driven by the fine adjustvoltage 203 e/212 b as discussed above (discussed in more detail below).The bias current sources of the AC amplifiers 504 are controlled inaccordance with the thermometer code represented by the bits b1, b2, b3,b4, b5, b6, b7 (bits b1-b6 for the high rate equalizers) of the coarsecontrol signals 125/129, while the bias current sources of the DCamplifiers 502 are driven by the inverses of such bits.

Referring to FIG. 11, an exemplary embodiment 504 a of the AC amplifiers504 include a tunable impedance implemented as an impedance 310 coupledbetween N-type metal oxide field effect transistors (N-MOSFETs) N1, N2,the gate electrodes of which are driven by the fine control voltage 203e/212 b. With the transistors N1, N2 operating in their linear operatingregions, a fine boost vernier control is provided, with step-wiselinearity (discussed in more detail below). The impedance 310 can beimplemented as virtually any form of impedance, such as a combination ofone or more additional resistances and one or more capacitances. Inaccordance with a preferred embodiment, the impedance 310 is implementedas a capacitance. As a result, in accordance with the thermometer-codedbits bn, the gain of the equalizer circuit 202 a/202 b/202 c/202 d/212 a(FIG. 10) will be as follows:

b1 b2 b3 b4 b5 b6 b7 Gain 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 + ω * R * C1 11 0 0 0 0 0 1 + ω * R * (C1 + C2) 1 1 1 0 0 0 0 1 + ω * R * (C1 + C2 +C3) 1 1 1 1 0 0 0 1 + ω * R * (C1 + C2 + C3 + C4) 1 1 1 1 1 0 0 1 + ω *R * (C1 + C2 + C3 + C4 + C5) 1 1 1 1 1 1 0 1 + ω * R * (C1 + C2 + C3 +C4 + C5 + C6) 1 1 1 1 1 1 1 1 + ω * R * (C1 + C2 + C3 + C4 + C5 + C6 +C7) for Zn = 1/(jω(0.5Cn) and n = number of stage

Referring to FIG. 12, as discussed above, the thermometer coding of thefine adjust bits provide for a step-wise linear adjustment of theequalization, with each of these 32 fine steps providing a resolution of0.08 dB between adjacent ones of the 768 coarse steps in the high ratefilter 102 and 224 coarse steps in the low rate filter 112.

Referring to FIGS. 13 and 14, the timing of the adjustment of the finecontrol voltage and coarse tuning bits are preferably as indicated. Forexample, adjustment of the fine tuning voltage should only occur whenthe AC amplifiers 504 are enabled and the DC amplifiers 502 aredisabled.

Referring to FIG. 15, an exemplary embodiment of the rate detectionstage 130 includes a high bandwidth band pass filter 230 a, a lowbandwidth band pass filter 230 b, full wave rectification circuits 230c, 230 d, a summing circuit 230 e, and integration circuitry 230 f,interconnected substantially as shown, similar to the adaptation stages106, 116 (FIG. 5). The high band pass filter 230 a provides a filteredsignal 231 a indicative of signal energy in the high frequency band,while the low band pass filter 203 b provides a filtered signal 231 bindicative of energy in the low frequency band. These signals 231 a, 231b are full-wave rectified by the rectification circuits 230 c, 230 d,and the rectified signals 231 c, 231 d are subtracted in the summingcircuit 230 e to produce a signal 231 e indicating whether the highfrequency band or low frequency band contains more energy. This signal231 e is integrated by the integration circuitry 230 f to produce therate detection signal 131. (In accordance with an alternativeembodiment, due to their similarities, with appropriate signal switchingand routing within the equalizer 100, the rate detection stage 130 canbe implemented by sharing filters, rectification circuits, summingcircuitry and integration circuitry with one or both of the adaptationstages 106, 116.)

Based upon the foregoing discussion, it will be understood that changesin equalization boosts will have some effect on the low frequency bandthat is used for amplitude control and calibration. Conversely, changesin the amplitude of the sliced signals 105, 115 will have some effect onthe energy in the high frequency boost adaptation. This effectivelyresults in two interacting loops that can potentially diverge and causethe equalization adaptation to go out of lock or convergence. However,this is avoided by operation of the finite state machine 220 b, whichuses interlaced amplitude and equalization boost loop adaptation andallows for disabling of the amplitude calibration loop. Early saturationof the amplitude calibration can be implemented to freeze the amplitudecalibration loop beyond a predetermined range. Additionally, aprogrammable timeout from the amplitude and equalization boosts loopsare different and separated in frequency. Further still, a programmabletimeout from the amplitude and equalization boost loops are differentand separated in frequency. Further still, a programmable timeout fromthe amplitude and equalization boost loops is used in case there is notoggling between the two loops for a predetermined time interval. Thisalso ensures that the loops do not remain stuck in a sub-optimalsolution.

The embodiments discussed hereinabove have been designed forimplementation by National Semiconductor Corporation as integratedcircuits for low power adaptive cable equalization. Copies of thepreliminary data sheets for two such implementations are included aspart of this disclosure (and are hereby incorporated herein byreference) in the form of Appendices A and B.

What is claimed is:
 1. An apparatus including circuitry for adaptivesignal equalizing with coarse and fine boost controls, comprising:differential amplifier circuitry responsive, in accordance with avariable signal gain, to at least one controlled supply current and adifferential input signal by providing a differential output signal;current source circuitry coupled to said differential amplifiercircuitry and responsive to at least a first control signal by providingsaid at least one controlled supply current; a variable impedancecoupled to said differential amplifier circuitry and responsive to atleast a second control signal by establishing said variable signal gain;wherein said variable impedance comprises a tunable impedance and afixed impedance mutually coupled; wherein said tunable impedance andfixed impedance are mutually coupled in parallel; wherein said tunableimpedance comprises a capacitance and at least one transistor mutuallycoupled; wherein said capacitance and at least one transistor aremutually coupled in series; and wherein said fixed impedance comprises aresistance.
 2. The apparatus of claim 1, wherein said differentialamplifier circuitry comprises first and second transistors coupled toreceive respective phases of said differential input signal and providerespective phases of said differential output signal.
 3. The apparatusof claim 1, wherein said current source circuitry comprises first andsecond current source circuits coupled to provide respective portions ofsaid at least one controlled supply current in accordance with said atleast a first control signal.
 4. An apparatus including circuitry foradaptive signal equalizing with coarse and fine boost controls,comprising: first differential amplifier circuitry responsive to atleast a first controlled supply current and a differential input signalby providing a first portion of a differential output signal; firstcurrent source circuitry coupled to said first differential amplifiercircuitry and responsive to at least a first control signal by providingsaid at least a first controlled supply current; second differentialamplifier circuitry coupled to said first differential amplifiercircuitry and responsive, in accordance with a variable signal gain, toat least a second controlled supply current and said differential inputsignal by providing a second portion of said differential output signal;second current source circuitry coupled to said second differentialamplifier circuitry and responsive to at least a second control signalby providing said at least a second controlled supply current; avariable impedance coupled to said second differential amplifiercircuitry and responsive to at least a third control signal byestablishing said variable signal gain; wherein said variable impedancecomprises a tunable impedance and a fixed impedance mutually coupled;wherein said tunable impedance and fixed impedance are mutually coupledin parallel; wherein said tunable impedance comprises a capacitance andat least one transistor mutually coupled; wherein said capacitance andat least one transistor are mutually coupled in series; and wherein saidfixed impedance comprises a resistance.
 5. The apparatus of claim 4,wherein said first and second control signals include substantiallymutually exclusive signal assertion states.
 6. The apparatus of claim 4,wherein said first differential amplifier circuitry comprises: first andsecond transistors coupled to receive respective phases of saiddifferential input signal and provide respective phases of said firstportion of said differential output signal; and a fixed impedancecoupled between said first and second transistors.
 7. The apparatus ofclaim 6, wherein said fixed impedance comprises a resistance.
 8. Theapparatus of claim 4, wherein said first current source circuitrycomprises first and second current source circuits coupled to providerespective portions of said at least a first controlled supply currentin accordance with said at least a first control signal.
 9. Theapparatus of claim 4, wherein said second differential amplifiercircuitry comprises first and second transistors coupled to receiverespective phases of said differential input signal and providerespective phases of said second portion of said differential outputsignal.
 10. The apparatus of claim 4, wherein said second current sourcecircuitry comprises first and second current source circuits coupled toprovide respective portions of said at least a second controlled supplycurrent in accordance with said at least a second control signal.